1. Field of the Invention
This invention relates to castors of the twin-wheel type, comprising two coaxial wheels mounted for independent rotation on opposite sides of a body which, when the castor is fitted, is rotatably mounted below the article to be supported. Normally, but not essentially, the body either turns on, or is fixed to, a vertical mounting spindle.
2. Description of the Prior Art
Castors conventionally require the provision of fixing means to locate the castor wheels axially with respect to, and retain them in, the castor bodies. Such means most commonly comprise spring rings which engage stub axles of the wheels which run in the bodies. Not only do the fixing means have to be produced as separate parts but fitting them complicates assembly, in many cases very considerably.
Single wheel castors are known (U.S. Pat. No. 3,991,434) in which the body mounting spindle engages a grooved stub axle of the wheel for positive retention of the latter, and although this has proved very successful in eliminating of separate fixing means it cannot be employed with a twin-wheel castor with independently rotatable wheels. In order to dispense with separate fixing means in a twin-wheel castor it has been proposed that the wheel axles should also have a snap-in fixing in the castor bodies, during assembly a moulded internal annular projection engaging a groove in the corresponding wheel axle, or vice versa. In this case, however, the body projection or groove is normally of such form that when moulded the body can snap off the moulding core. Thus there is the inherent disadvantage that positive wheel retention cannot be achieved, as in service the application of side loading can detach a wheel at the same force as the snap-off force during moulding. Furthermore, a long moulding cycle time is required as the moulding must cool and harden sufficiently to avoid damage to the moulded projection or groove when snapping off the mould core.
In order to overcome the foregoing disadvantages it has been proposed (Canadian Patent No. 780110) that the castor body should have an intermediate centrally disposed opening from opposite sides of which separate sections of the wheel mounting bore extend to receive the wheel axles. This enables wheel-retaining annular projections to be moulded at the inner ends of the bore sections with the desired rectangular profile for positive retention as it is no longer necessary for the projections to snap off a moulding core. However, this is only achieved at the considerable expense of employing three-way core withdrawal during the moulding operation, and considerable weakening of the body due to the necessity for the central opening at a highly stressed point where strength is most to be desired.